One very confusing aspect of lubricating bearings properly is knowing how much grease should be added to a bearing. Too little grease will leave a bearing with inadequate lubrication and associated overheating and premature wear. Too much grease will damage bearing seals allowing contaminants to enter and grease to escape. Adding the wrong amount of grease can be as harmful as not greasing bearings at all.
There are three, commonly used, rules of thumb for greasing sealed bearings. Not all bearings are sealed. There are bearings that have a discharge port to let old grease out while while new grease is being added. However, sealed bearings are much more common.
Rule of Thumb #1: G = DB/10
This rule of thumb uses the formula G-DB/10 where D=shaft diameter (in.), B=bearing width (in.), and G=ounces of of grease. To use this (or the next) rule of thumb, you will first need to “calibrate” your grease gun. This is done by pumping a known number of strokes on your grease gun and weighing the discharged grease on a scale.
As an example of using this formula, let’s consider a sealed bearing on a 2″ diameter shaft that is 7/8 of an inch wide. (2 x .825)/10 = 0.16 oz of grease.
Rule of Thumb #2: One Stroke per Inch of Shaft
This rule of thumb depends entirely on the grease gun. Different grease guns deliver different volumes of grease per stroke. It would probably be better to say 2 or 3 grams per inch of shaft diameter. Most grease guns deliver a volume close to this figure. Before trusting this rule of thumb, you need to know your grease gun. 10 strokes of grease weighted on an accurate scale will tell you how much grease you are delivering per stroke of the grease gun.
Rule #3 – Feel and Listen
It’s possible to tell when a bearing is properly lubricated by listening to the bearing and feeling the pressure via the grease gun handle. To hear the differences in the bearing sounds, the bearing must be rotating at operating speed. A mechanic’s stethoscope can make this easier. When the first shot of grease is injected, the bearing noise will usually decrease significantly as the old, dry grease is displaced by the new grease. As grease is added, the bearing noise will remain nearly constant with brief changes corresponding to each stroke of the grease gun. These brief changes subside quickly after each stroke. When the bearing finally becomes “full”, the sound level will begin to increase as grease is added. Grease should only be added to the bearing until this increase in noise starts to occur.
At the same time that the bearing becomes “full”, an increase in grease pressure occurs. The lubrication tech should be conscious of the resistance felt in the grease gun handle. As the pressure begins to increase, the increase in sound should correspond to an increase in felt resistance.
However, it is important to be aware that this procedure changes when using a grease extension hose. Because grease extension hoses can expand slightly under increased grease pressure, and because grease is extremely viscous, grease moves slowly through these hoses. The delay between adding grease and that grease reaching the bearing is only a few seconds with short hoses under three feet. As hose length increases, this delay can become significant.
Disclaimer: Every condition found in the field is unique. Rules of thumb and best practices described here may not work in all situations. We recommend consulting an engineer or lubrication specialist for advice on your specific situation.